Grain Refinement Of Mg Alloys Research Articles

Grain refinement of Mg-Ca alloys by native MgO particles

In Mg-Ca alloys the grain refining mechanism, in particular regarding the role of nucleant substrates, remains the object of debates. Although native MgO is being recognised as a nucleating substrate accounting for grain refinement of Mg alloys, the possible interactions of MgO with alloying elements that may alter the nucleation potency have not been elucidated yet. Herein, we design casting experiments of Mg-xCa alloys varied qualitatively in number density of native MgO, which are then comprehensively studied by advanced electron microscopy. The results show that grain refinement is enhanced as the particle number density of MgO increases. The native MgO particles are modified by interfacial layers due to the co-segregation of Ca and N solute atoms at the MgO/Mg interface. Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy, we reveal the nature of these Ca/N interfacial layers at the atomic scale. Irrespective of the crystallographic termination of MgO, Ca and N co-segregate at the MgO/Mg interface and occupy Mg and O sites, respectively, forming an interfacial structure of a few atomic layers. The interfacial structure is slightly expanded, less ordered and defective compared to the MgO matrix due to compositional deviations, whereby the MgO substrate is altered as a poorer template to nucleate Mg solid. Upon solidification in a TP-1 mould, the impotent MgO particles account for the grain refining mechanism, where they are suggested to participate into nucleation and grain initiation processes in an explosive manner. This work not only reveals the atomic engineering of a substrate through interfacial segregation but also demonstrates the effectiveness of a strategy whereby native MgO particles can be harnessed for grain refinement in Mg-Ca alloys.

Grain refinement of Mg-alloys by native MgO particles: An overview

A fine and equiaxed solidification process delivers multidimensional benefits to Mg-alloys, such as improved castability, reduced casting defects, enhanced mechanical properties, increased corrosion resistance and potential for increased recycled contents. Despite extensive research on grain refinement of Mg-alloys in the last few decades, currently, there is no effective grain refiner available for refining Mg-Al alloys, and our current understanding of grain refining mechanisms is not adequate to facilitate the development of effective grain refiners. Under the EPSRC (UK) LiME Hub's research program, substantial advances have been made in understanding the early stages of solidification covering prenucleation, heterogeneous nucleation, grain initiation and grain refinement. In this paper, we provide a comprehensive overview of grain refinement of Mg-alloys by native MgO particles. We show that native MgO particles can be made available for effective grain refinement of Mg-alloys by intensive melt shearing regardless of the alloy compositions. More importantly, we demonstrate that (1) the addition of more potent exogenous particles will not be more effective than native MgO; and (2) MgO particles are difficult to be made more impotent for grain refinement through promoting explosive grain initiation. We suggest that the most effective approach to grain refinement of Mg-alloys is to make more native MgO particles available for grain refinement through dispersion, such as by intensive melt shearing.

A Study on the Grain Refining Mechanisms and Melt Superheat Treatment of Aluminum-Bearing Mg Alloys

Grain refinement of magnesium (Mg) alloys has been a major research topic over the past decades as one of the effective approaches to increase their strength and ductility simultaneously. In this study, a brief review of the grain refinement of aluminum-bearing Mg alloys is included to provide an in-depth understanding of the detailed mechanisms of grain refinement of Mg alloys. Additionally, the effect of melt superheating on the grain refining of Mg–Al-based alloys has been investigated. It was confirmed that melt superheating caused a significant grain refining effect in the commercial purity (CP) of AZ91 alloy (0.25% Mn). Undercooling of 1.3 °C was observed before superheating and was noticeably reduced after the superheating process. A vacuum filtering experiment was conducted, which involves filtering the melts using fine metal porous filters to separate the particles in the melts. It was observed that a large amount of Al8Mn5 particles were generated in the commercial purity AZ91 alloy by the superheating process. However, because of the poor crystallographic matching between Al8Mn5 and Mg, Al8Mn5 was not considered the nucleation site for Mg grains. A master alloy containing ε-AlMn particles, which are in good crystallographic matching with Mg, was added, and it was found that the grain size of the commercial-grade AZ91 alloy was reduced. Therefore, it is suggested that Al8Mn5 particles, existing as a solid phase in the molten metal of the commercial AZ91 alloy could be transformed into ε-AlMn particles by the superheating process, and these particles can be effective nucleation sites for Mg grains. The transformation of Al8Mn5 into ε-AlMn is considered the main mechanism of grain refinement of the commercial purity of AZ91 alloy by superheating. Notably, the effect of grain refinement by superheating was not observed in the high-purity (HP) AZ91 alloy (0.006% Mn) because Al–Mn particles were likely not formed due to a very small quantity of manganese.

Characterization of AZ91 magnesium alloy processed by cyclic contraction/expansion extrusion using the experimental and micromechanical cellular automaton finite element approach

The feasibility to fabricate ultra-fine-grained AZ91 Mg alloy is investigated with a newly presented severe plastic deformation method entitled cyclic contraction/expansion extrusion. In this method, an initial cylindrical AZ91 Mg billet is placed into a die and the moving punch causes the large deformations by extruding the material in two different regions entitled contraction and extrusion. The evolution of AZ91 microstructure and mechanical properties during cyclic contraction/expansion extrusion was studied through different experimental observations. The microstructure observations showed the ultra-fine-grained structure of AZ91 at the end of the third pass where the average grain size of 600 nm obtained from the initial value of 130 µm. The tensile tests showed that the ultimate tensile strength, yield strength, hardness, and elongation of AZ91 cyclic contraction/expansion extrusion-processed samples are increased significantly. Discontinuous dynamic recrystallization has a main role in the grain refinement of Mg alloys during hot deformations. The evolution of grains in microlevel is analyzed by the cellular automaton finite element method in the DEFORM software environment. The macroscopic flow parameters including effective plastic strain, stain rate, and temperature were calculated in finite element. By tracing these data in defined domain of cellular automaton, the discontinuous dynamic recrystallization of material is analyzed through a devised cellular automaton finite element post-processing step. The imposed plastic strain and variation of dislocation density are the two main driven forces in discontinuous dynamic recrystallization of AZ91 samples during cyclic contraction/expansion extrusion processing. The experimentally observed grains and the cellular automaton finite element predicted microstructure were reasonably in good agreement.

Prenucleation at the Interface Between MgO and Liquid Magnesium: An Ab Initio Molecular Dynamics Study

Magnesia (MgO) particles inevitably exist in liquid Mg and may be used as potential sites for heterogeneous nucleation to achieve effective grain refinement. Understanding of the atomic configurations on MgO surfaces and in the liquid Mg adjacent to the liquid Mg/MgO interfaces is therefore of both scientific and practical interests. We investigate the surface structures of MgO in liquid Mg and the atomic arrangements of liquid Mg adjacent to liquid/substrate interfaces, using an ab initio molecular dynamics (MD) simulation technique. We find that an atomically rough terminating Mg layer forms on the {1 1 1} terminated MgO substrate (octahedral MgO) in liquid Mg. The simulations also reveal that on the structurally flat {0 0 1} terminated MgO substrate (cubic MgO) a rough Mg layer forms due to the unique chemical interactions between the ions on the substrate and the liquid metals. The surface roughness together with the large lattice misfits with solid Mg makes both octahedral and cubic MgO substrates impotent for heterogeneous nucleation of α-Mg. The present results may shed new light on grain refinement of Mg-alloys.

Segregation of Ca at the Mg/MgO interface and its effect on grain refinement of Mg alloys

It has been reported that native MgO particles in Mg alloy melts can act as heterogeneous nucleation substrates such that grain refinement of Mg alloys is achieved. A recent study showed the addition of Ca, combined with the native MgO particles, significantly improves grain refinement of Mg and its alloys. However, the mechanism underlying the grain refining phenomenon is not well understood due to the lack of direct experimental evidence. In this work, we investigated the segregation of Ca atoms at the Mg/MgO interface and its effect on grain refinement in Mg-0.5Ca alloys by utilizing advanced analytical electron microscopy. The experimental results focus on the chemical and structural information at the interface between MgO and the Ca solute. Adsorption layers rich in Al, N and Ca have been detected on {1 1 1} facets of MgO particles, with the lattice structure resembling the structure of MgO. It is suggested that the significant grain refinement improvement can be attributed not only to the growth restriction due to the presence of Ca addition but also to the specific chemistry and structure of the adsorption layers.

Discussion on grain refining mechanism of AM30 alloy inoculated by MgCO3

Abstract AM30 was inoculated by MgCO 3 powder with different holding time. The influence of MgO decomposed by MgCO 3 on the grain refinement effect was mainly discussed in the present study. Three sets of comparative samples were prepared. They were AM30 alloy inoculated by MgO and pure Mg inoculated by MgO and MgCO 3 . The possible nucleating particles were observed and analyzed by EPMA and SEM. AM30 alloy could be effectively refined by either MgCO 3 or MgO inoculation. Grain refining efficiency and fading effect of MgO inoculation were better than those of MgCO 3 inoculation. However, pure Mg could not be refined by these two inoculants. Al is an indispensable element to determine the grain refinement of Mg alloys inoculated by either MgCO 3 or MgO. MgO should not be the effective substrates for α-Mg phase. A novel grain refining mechanism of MgCO 3 inoculation on AM30 alloy was proposed by combining experimental results with theoretical calculation, i.e., MgAl 2 O 4 should be the potent nuclei of α-Mg grain for the AM30 alloy in addition to Al 4 C 3 .

Effect of sonotrode material on grain refining of Mg–3Al and Mg–9Al alloys by ultrasonic melt treatment

The new process, nucleation enhanced ultrasonic melt treatment (NEUMT), was proposed to increase the refining efficiency through heterogeneous nucleation by using the sonotrode which has been only concerned with the medium to transfer the ultrasonic energy. In the processing, the metal atoms and/or clusters eroded from the sonotrode were supplied and were simultaneously mixed uniformly into the melt by the ultrasound. These particles act as potential nuclei and refine the structure. The process was applied to assess grain refinement of Mg alloys, especially Mg–3Al and Mg–9Al. The refining efficiency was affected by the sonotrode material, and Ti was very effective in this process by the formation of proper intermetallic compound in the Mg alloy melt. The intermetallic compound was searched by the calculation of plane disregistry of the crystallographic orientation, and Al3Ti was suggested to be the heterogeneous nuclei.

Study of Soft Impingement of Diffusion Fields and Its Effect on Cast Microstructure

Rigorous Mullins and Sekerka perturbation theory has been applied to two solidification interfaces. The results indicate that as the distance between the two interfaces decreases, the rate of amplitude growth of perturbations on the planar solid–liquid interface becomes negative, which is a further verification of the stability of a planar interface due to soft impingement of diffusion fields during solidification. This work conclusively shows that spheroidal grains form due to grain refinement of Mg alloys by Zr addition because of the high density of nucleation.

Grain Refinement of Magnesium Alloys: A Review of Recent Research, Theoretical Developments, and Their Application

This paper builds on the “Grain Refinement of Mg Alloys” published in 2005 and reviews the grain refinement research on Mg alloys that has been undertaken since then with an emphasis on the theoretical and analytical methods that have been developed. Consideration of recent research results and current theoretical knowledge has highlighted two important factors that affect an alloy’s as-cast grain size. The first factor applies to commercial Mg-Al alloys where it is concluded that impurity and minor elements such as Fe and Mn have a substantially negative impact on grain size because, in combination with Al, intermetallic phases can be formed that tend to poison the more potent native or deliberately added nucleant particles present in the melt. This factor appears to explain the contradictory experimental outcomes reported in the literature and suggests that the search for a more potent and reliable grain refining technology may need to take a different approach. The second factor applies to all alloys and is related to the role of constitutional supercooling which, on the one hand, promotes grain nucleation and, on the other hand, forms a nucleation-free zone preventing further nucleation within this zone, consequently limiting the grain refinement achievable, particularly in low solute-containing alloys. Strategies to reduce the negative impact of these two factors are discussed. Further, the Interdependence model has been shown to apply to a broad range of casting methods from slow cooling gravity die casting to fast cooling high pressure die casting and dynamic methods such as ultrasonic treatment.

Production of ultrafine-grain microstructure in Mg alloy by alternate biaxial reverse corrugation

Magnesium alloys usually have poor formability at ambient temperatures due to a lack of sufficient number of slip systems associated with hexagonal close-packed crystal structure. Twinning is often found to accommodate plastic deformation in addition to slip in Mg alloys. Grain refinement of Mg alloys can significantly alter these processes. In the present work, severe plastic deformation is imparted to Mg alloy plate AZ31B using a process of alternate biaxial reverse corrugation followed by flattening. By progressively decreasing the deformation temperature from 250 to 170 °C during repeated deformation, an initial bimodal grain structure is progressively subdivided by slip and twinning. Concurrent dynamic recovery and some amount of recrystallization lead to the formation of nearly uniform ultrafine microstructure of average size 1.4 μm at ε = 5.0. Twinning deformation causes grain refinement in the early stages, but as the grain size becomes finer twinning is increasingly inhibited even though the deformation temperature is lowered. Deformation twins disappear when the average grain size reaches about 2–3 μm. Texture evolution during processing is seen to be affected by the amount of strain and instantaneous grain size. A strong basal texture is formed after several flattening processes, but its intensity drops again as the grain size becomes finer.